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Thread: CAD Design v.s CNC Machining

  1. #1
    Associate Engineer
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    CAD Design v.s CNC Machining

    Hello everyone,

    I am new to this forum so let me introduce myself. I recently started helping my brother run his CNC Machine Shop. He gave me the task of doing the QC Inspection on parts and I am learning a lot. My job is to please the Engineers( you guys), but it's not an easy job. So here I am trying to understand really what it is that the engineers want. I have a few questions. And I don't mind answering any questions that maybe a few of you might have for me as well. I think this can be a win-win discussion. I can also tell you how we(cnc machinist) feel about mechanical engineers. Disclaimer: I just started this job so please excuse anything I say that is wrong.


    1. We are provided both CAD drawings and STEP files. But sometimes they don't match up. My brother say ultimately the STEP dimension is more correct than the CAD drawing. This can be confusing as I am writing my CMM program to match up with the dimensions on the drawing but the CMM is telling me the dimension on the STEP file is off. What is going on your side that might be causing this? Or is this just the engineer's fault?

    2. We have drawings from a client that call out A LOT of true positions, which my brother said I can just ignore since it is very hard to achieve. There is a thing call propagated error. The parts are slightly out of its tolerance of true position and the parts are never rejected. Which reconfirm my brother's judgement, you guys don't really care. Do you guys really care about true positions or is this just a rookie's job thinking he can make the part as perfect as he want?

    3. We get parts that may look simple, but to the CNC machinist's perspective can be very tricky. Do Mechanical Engineers have any training at all on what is capable from a CNC machine? We had a part that cost $900/EA just to make and it is just an aluminum frame.

    4. Just any tip/advice for me in general to please our customers( you engineers). Thanks.

  2. #2
    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by tupham View Post
    Hello everyone,
    My job is to please the Engineers( you guys), but it's not an easy job. So here I am trying to understand really what it is that the engineers want.
    No, this is NOT your job. You are to verify that the end part is manufactured to the specifications on the engineering drawing - nothing less, nothing more.

    Quote Originally Posted by tupham View Post
    1. We are provided both CAD drawings and STEP files. But sometimes they don't match up. My brother say ultimately the STEP dimension is more correct than the CAD drawing. This can be confusing as I am writing my CMM program to match up with the dimensions on the drawing but the CMM is telling me the dimension on the STEP file is off. What is going on your side that might be causing this? Or is this just the engineer's fault?
    The 3D step file should match the engineering drawing. There's not fault here the task you and your brother have is to ensure that your customer gets the the part built to specification. If there is a difference between the CAD model and engineering drawing you should default to the engineering drawing or whatever you quoted too. If your quote specified that you were to build to the CAD file and inspect/specify to the engineering drawing then you should require a matching CAD step file. Ultimately, you should flag the responsible engineer or designer to let them know there id a difference between the engineering drawing and the CAD file. Also get in writing what the expectations are going forward.


    Quote Originally Posted by tupham View Post
    2. We have drawings from a client that call out A LOT of true positions, which my brother said I can just ignore since it is very hard to achieve. There is a thing call propagated error. The parts are slightly out of its tolerance of true position and the parts are never rejected. Which reconfirm my brother's judgement, you guys don't really care. Do you guys really care about true positions or is this just a rookie's job thinking he can make the part as perfect as he want?
    Ignore the engineering drawing specifications? Don't, chances are that your customer is not verifying the incoming parts from your organization, reworking or accepting as-is. Either way, you guys are NOT doing the job you agreed too and it will bite you eventually. If you can't do it right - don't do it, deliver what you promised.

    When I encounter customer who tell me manufacturing does not understand GD&T - I tell them they have the wrong manufacturing organization or people - move on.


    Quote Originally Posted by tupham View Post
    3. We get parts that may look simple, but to the CNC machinist's perspective can be very tricky. Do Mechanical Engineers have any training at all on what is capable from a CNC machine? We had a part that cost $900/EA just to make and it is just an aluminum frame.
    No, most designers and engineers have no experience in machining. Moreover, most do not understand the manufacturing process capability nor do they consider it during the design process. I'll go a step further and say most do not even understand the mechanical tolerances required for their end-items to function.

    I just happen to be a DFM/DFA as well as GD&T consultant and trainer BTW.

    Quote Originally Posted by tupham View Post
    4. Just any tip/advice for me in general to please our customers( you engineers). Thanks.
    Step up your game, quote what you can build avoid what you cannot. Do what you say you're going to do, quality, quality, and more quality. Deliver on time, at the right price. You're in a competition with the whole world don't fall to last place.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  3. #3
    Technical Fellow jboggs's Avatar
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    I feel your pain my friend. I've been on the engineering side for several decades, and I can tell you - just like most professions, we have more than our share of sloppy engineers. Everything I know about machining I learned AFTER graduating, thru the school of hard knocks.

    I have also learned a few other things. The most important of which is this - I am not God. If I want one of my designs to be accepted and work well, I have to have the buy-in of both the folks that will make it and the folks that will use it. I do not mind questions. I welcome, even encourage them. I learn from every one and improve myself.

    Sadly one of the things I have seen FAR TOO OFTEN, especially from younger engineers, is that they are so afraid of their designs not working that they throw a lot of unnecessary requirements on there without even truly understanding what they mean, just to cover their own ass. In their minds, the more "stuff" on the drawing the better it is. They don't really know if a particular hole truly has to be in a certain location within .001", but they know that if it isn't they might get blamed for something, so they throw yet another true position requirement on there. Very easy solution, right?

    I would advise you and your brother to quote the jobs as drawn, make them to print, and charge these yahoos a realistic price for hwat it takes to do that. By deciding on your own which of those excessive requirements are needed and which aren't, you are also taking the liability of missing one that really is needed. Then it becomes your fault.

    As far as drawings not matching the STEP files, that is sloppy engineering, pure and simple. If they do not match, I would recommend that you advise your client that you really cannot proceed with conflicting requirements. It is their job to decide what's important and fix the discrepancy, not yours.

    I can also tell you this - just because you are seeing certain problems with the engineers that designed the items your shop is building, do not make the mistake of thinking that all engineers are like that. They aren't.

  4. #4
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    Thanks for the reply. Quote the job as drawn, easier said than done. This is a game we are playing. To beat the competition we have to quote smart. And the engineers want their parts in less than a week. Sometimes we have to be flexible, turn in the part, hope they can use it.

  5. #5
    Principle Engineer Cragyon's Avatar
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    Quote Originally Posted by tupham View Post
    Thanks for the reply. Quote the job as drawn, easier said than done. This is a game we are playing. To beat the competition we have to quote smart. And the engineers want their parts in less than a week. Sometimes we have to be flexible, turn in the part, hope they can use it.
    Not sure what you came here for, seems like looking for an endorsement to not do the job right. What you seem to be doing here is putting money before honor, integrity and before your customers requirements. You will get bit.



    "Sometimes we have to be flexible, turn in the part, hope they can use it.".

    This why US companies outsource to those other places in the world.

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